Shale development requires companies to truck in water. Those trucks burn a lot of fuel. And natural gas, which is basically methane, is a potent greenhouse gas if it leaks from the well heads or pipelines. Also, chemicals used to fracture the rock have been known to spill contaminating local water supplies.

"It is not at all clear that gas is clean," said Robert Howarth, a Cornell ecologist who is studying the issue. "A complete consideration of all emissions from using natural gas seems likely to make natural gas far less attractive than oil and not significantly better than coal."

But conventional wisdom still holds that gas is cleaner. Maybe not the 50% cleaner that it was once thought, but still significantly better than coal, the mining of which often involves open pits, strip mines or the removal of entire mountain tops, said Dave Hamilton, director for global warming and energy projects at the Sierra Club.

The real questions is this: Can shale gas be developed economically while still protecting the ground water?

The best new wells in Pennsylvania can produce natural gas for about $4 per million BTUs, said Robert Ineson, senior director for global gas at IHS Cambridge Energy Research Associates.

So long as natural gas remains below the $6 BTU range, utility analysts say it will be competitive with other energy sources like wind or coal.

Room for profit. Kevin Book, an analyst at ClearView Energy Partners, said regulators could require greater treatment of the fluids used to drill a well and place tighter restrictions on where companies get water.

They could also impose stricter standards for the casing that lines a well hole, expand ground water monitoring and give the federal government a greater role in this process.

All this, said Book, might add between 15 cents and 75 cents per BTU to production costs.

That means there is room for both profit and additional regulation.

Or, as Cambridge's Ineson puts it, additional regulations are "warranted, likely and affordable. You're not going to kill the shale boom on the basis of these costs."

Pennsylvania has recently enacted some of these additional regulations, but not all of them.

Environmentalists are calling for more. They want 3D seismic imaging of drilling areas to detect cracks where the gas could seep into. They also want old wells to be plugged.

It's not known exactly how much these other measures would cost. At some point a balance must be reached between producing shale gas economically and protecting the environment. The hard part is finding it.